Attacks can be triggered by car fumes

Dr Ira Tager, of the University of California at Berkeley, told New Scientist magazine: "It fits well with what's known in humans about developmental effects."

But Dr Adnan Custovic, a National Asthma Campaign senior research fellow and an honorary consultant allergist at Wythenshawe Hospital in Manchester, said the weight of evidence pointed away from air pollution as a cause of asthma.

Plenty of studies had found no link between air pollution and levels of asthma, he said.

A recent study comparing children in heavily air-polluted former East Germany to those in the west following reunification found that asthma rates were actually lower in the east.

He added: "The problem with animal models of asthma is that you don't have one that is really representative of human asthma.

"That is why this kind of research is so difficult.

"It seems to be very clear that air pollution has a lot to do with triggering asthma attacks, but is not actually a cause."

In fact, he pointed out, asthma cases have soared since the introduction of the Clean Air Act - which radically decreased the amount of air pollution in the UK over the following decades.